The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain particulate absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" materials has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such particulate absorbent polymers in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these particulate absorbent polymers to absorb large quantities of discharged aqueous body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and particulate absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
These particulate absorbent polymers are unsurpassed in their ability to retain large volumes of fluids, such as urine. A representative example of such particulate absorbent polymers are lightly crosslinked polyacrylates. Like many of the other absorbent polymers, these lightly crosslinked polyacrylates comprise a multiplicity of anionic (charged) carboxy groups attached to the polymer backbone. It is these charged carboxy groups that enable the polymer to absorb aqueous body fluids as the result of osmotic forces.
Besides osmotic forces, absorbency based on capillary forces is also important in many absorbent articles, including diapers. Capillary forces are notable in various everyday phenomena, as exemplified by a paper towel soaking up spilled liquids. Capillary absorbents can offer superior performance in terms of the rate of fluid acquisition and wicking, i.e. the ability to move aqueous fluid away from the point of initial contact. Indeed, the dual-layer core absorbent structures noted above use the fibrous matrix as the primary capillary transport vehicle to move the initially acquired aqueous body fluid throughout the absorbent core so that it can be absorbed and retained by the particulate absorbent polymer positioned in layers or zones of the core.
An alternative absorbent material potentially capable of providing capillary fluid transport would be open-celled polymeric foams. If made appropriately, open-celled polymeric foams could provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores for absorbent articles such as diapers. Absorbent articles containing such foams could possess desirable wet integrity, could provide suitable fit throughout the entire period the article is worn, and could avoid degradation in shape during use. In addition, absorbent articles containing such foam structures could be easier to manufacture on a commercial scale. For example, absorbent diaper cores could simply be stamped out of continuous foam sheets and could be designed to have considerably greater integrity and uniformity than air-laid fibrous absorbent cores containing particulate absorbent polymers. Such foams could also be molded in any desired shape, or even formed into integral, unitary diapers.
Literature and commercial practice is replete with descriptions of various types of polymeric foams that can imbibe a variety of fluids for a variety of purposes. Indeed, employment of certain types of polymeric foam materials as elements of absorbent articles such as diapers and catamenial products has previously been suggested. See, for example, U.S. Pat. No. 4,029,100 (Karami), issued Jun. 14, 1977, that discloses a shape-retaining diaper that can employ a foam element in the crotch area of its absorbent pad assembly in order to provide high wet resiliency. Certain types of polymeric foam materials have also been suggested as useful in absorbent articles for the purpose of actually imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,528 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structures such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams).
Although various polymeric foam materials have been suggested for use in absorbent articles, there is still a need for absorbent foam materials having optimized combinations of features and characteristics that would render such foams especially useful in commercially marketed absorbent products such as diapers. In terms of desired absorbency characteristics, including capillary fluid transport capability, it has been determined that optimized absorbent, open-celled polymeric foams should have the following characteristics:
(a) a relatively greater affinity for absorbing body fluids than exhibited by other components in the absorbent article so that the foam material can drain (partition) fluids from these other components and keep the fluids stored within the foam structure;
(b) relatively good wicking and fluid distribution characteristics in order for the foam to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure, thus allowing for subsequent gushes of fluid to be accommodated; and
(c) a relatively high storage capacity with a relatively high fluid capacity under load, i.e. under compressive forces.
As previously noted, a thinner absorbent core is usually a requirement for making relatively thin absorbent articles, such as diapers. However, providing absorbent polymeric foam materials that remain relatively thin in form until wetted with aqueous body fluids is not straightforward. The absorbent foam material needs to remain relatively thin during normal storage and use prior to being wetted. This relatively thin polymeric foam material must additionally have the needed absorbency characteristics described above if it is to be useful in high performance absorbent cores. Making relatively thin polymeric foams that are sufficiently soft and flexible for comfort of the wearer is also not a trivial task.
Accordingly, it would be desirable to be able to make an open-celled absorbent polymeric foam material that: (1) has adequate or preferably superior absorbency characteristics, including capillary fluid transport capability, so as to be desirable in high performance absorbent cores used in absorbent articles such as diapers, adult incontinence pads or briefs, sanitary napkins and the like; (2) is relatively thin during normal storage and use until wetted with aqueous body fluids; (3) is sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article; and (4) can be manufactured on a commercial scale, at relatively reasonable or low cost.